The present invention involves a metal connector plate for interconnecting members such as the wooden members forming a structural joint. The invention also involves the method and apparatus for forming such connector plates. Typically, such connector plates are utilized for interconnecting adjacent wooden members for forming a structural joint such as a roof or floor truss structure as shown in U.S. Pat. No. 2,877,520 to Jureit.
When interconnecting such wooden members, the two members are arranged so as to be butted together and a connector plate is either pressed or rolled into such abutting members. If a pressing action is used, the connector plate is arranged so as to sit on top of the wooden members and then a pressing force is applied. In the alternative action, the connector plate is rolled into the wood such as shown in U.S. Pat. No. 3,479,920 to Sanford. While it has been common practice for several years to roll connector plates for forcing them into the wood, such action has lead to several deficiencies in the final product as compared to the pressed connector plates. During the rolling action, there is a tendency for the connector plate to slip until it is firmly secured to the wood. Additionally, the rolling force applies both a force perpendicular to the plate and a longitudinal force to the plate. This longitudinal force can and often did lead to bending of the teeth of the connector plates until the teeth were firmly secured by a sufficient depth in the wood. Such bending, typically referred to as a submarining effect, was especially prevelant in those situations where the teeth met with any significant resistance against penetration from the wood. Where such a submarining effect becomes significant, the teeth do not penetrate the wood as deeply as desired thereby reducing the effective strength of the structural joint.
Another problem that occurs when rolling the connector plates for forcing them into the wooden members occurs since the force of the large roller that is typically utilized has a tendency to cause the connector to conform to the shape of the roller. The curving of the plate means that the teeth when contacting the wood will be oriented at an inclined angle instead of being perpendicular to the wood. This effect increases the tendency for bending or submarining of the teeth when they enter the wooden members. This problem has been long recognized in the prior art and in fact is discussed in the above noted Sanford patent, U.S. Pat. No. 3,479,920, which discloses a connector plate specially designed for roller presses.
The strength of the structural joint is typically measured with respect to the capability of the connector plate to resist withdrawal from the wooden members when subjected to longitudinal stress and the resistance provided by the connector plate against rotation of the wooden members. While conventional plates have had a high holding capability against longitudinal forces applied along the axis of the abutted wooden members, the holding capability against rotation of the wooden members has been lower. In attempting to improve the resistance against rotation U.S. Pat. No. 3,951,033 to Moehlenpah discloses a connector plate having a plurality of pairs of teeth. The connector plate is provided with a first group of such pairs of teeth oriented in one direction and pairs of teeth oriented in a different direction. Hydro-Air Engineering, Inc. markets its P-T truss plates utilizing the designation of U.S. Pat. No. 3,951,033. FIG. 8 illustrates a perspective view of a portion of a Hydro-Air P-T plate.
The Hydro-Air connector plate, both as shown in the patent to Moehlenpah and the plate itself, has a plurality of pairs of teeth with each pair being cut from a single slot. Each tooth is twisted so as to have one section that extends further forward than the remainder of the tooth. The thickness of each tooth is substantially constant and the forward edge has a cross-sectional angle of approximately 45.degree.. While not illustrated in the patent, in the Hydro-Air plate marketed under the patent, each tooth is twisted so as to actually lean either to the right or left, i.e., the teeth are not aligned with the slots, such as shown in FIG. 8.
The twisting of the teeth so as to lean in one or the other direction is a common phenomenon that will occur when air cutting the teeth from the metal plate. Such an air cutting process is illustrated in U.S. Pat. No. 3,685,336 to Black. While the patent to Black does show a typical air cutting process for cutting the teeth out of the plate, it does not illustrate the twisting of the nails that will typically inherently occur. Such twisting of the nails which causes them to lean in one or the other direction will lead to a further bending of the teeth in the same direction as the connector plate is rolled into the wooden members. Such bending of the nails leads to a submarining effect which will often weaken the structural joint. In essence, such a submarining effect means that the teeth are not embedded in the wood to the fullest possible extent but are bent over so as to be positioned relatively close to the surface of the wood thus being more easily withdrawn when the wooden members are placed under load. The effective result of such submarining effect on the strength of the joint will greatly vary in dependence upon the type and nature of the wood used in forming the joint. Thus, for example, in harder woods the problem is more significant.
In addition, the relatively blunt angle of the teeth of the Hydro-Air plate and as illustrated in the patent to Moehlenpah generates a greater resistance to entry of the teeth into the wood. As previously discussed such resistance to entry causes a shifting of the plate and further bending of the teeth thereby increasing the submarining effect.
Furthermore, due to the blunt angle of the edges of the teeth illustrated in the Moehlenpah patent and in the Hydro-Air plate, there is a compressing action on the wood grain when such teeth are rolled into the wooden members. Such compressing of the wood creates a tendency for the wood to collapse thereby creating a hollow space under the plate around the teeth and weakening the grip of the wood on the teeth and the structural strength of the joint.
During such an air cutting operation in the production of the connector plates, the space between the end walls of the punch and the inner end walls of the die cavity has always been greater than the thickness of the metal so that a free space remains after the teeth have been struck from the metal and pushed into the cavity. If the space was not larger than the thickness of the metal then friction would be generated as the metal was pressed against the inner end walls of the cavity thereby generating heat which could lead to a change of the shape of the die by expansion and contraction, warp the die or also possibly break the die or punch. Thus, the prior art has avoided such problems by allowing extra space in the area of the end walls of the punch and die cavity.
Another patent illustrating a connector plate having a plurality of pairs of teeth with a first group of such pairs oriented in one direction and the remainder of the teeth oriented in the other direction although the slots extend in a longitudinal direction is U.S. Pat. No. 3,603,197 to Wood. The patent to Wood illustrates a connector plate having a plurality of pairs of teeth that are substantially similar and in many ways identical to the teeth and connector plate illustrated in the above-noted patent to Moehlenpah. Other patents showing pairs of teeth cut from a connector plate with the teeth oriented in different directions are U.S. Pat. Nos. 3,094,748; 3,104,429; and 3,211,043, all to Sanford.
The connector plates are made by passing a metal sheet material in incremental steps between a set of punch and corresponding dies. The sheet material is stricken by the punches which cut the teeth and proceed to enter into the cavities of the dies so as to produce the teeth. Such procedures and the apparatus for carrying out such procedures are illustrated in U.S. Pat. Nos. 3,314,271 to Otis and 3,685,336 to Black. Both of these patents are hereby incorporated by reference. As previously discussed, the patent to Black illustrates an air cutting procedure for producing a pair of teeth struck from each slot. As shown in the patent to Black, when the punch is inserted into the cavity of the die, the distance between the punch and the inner surfaces of the die cavity is always greater than the thickness of the metal sheet material being punched. Thus, the teeth are produced inside of the cavity without actually contacting or being forced against the cavity walls.